This invention relates to apparatus for continuously producing photovoltaic devices by depositing successive layers of semiconductor material onto a substrate as that substrate travels through operatively connected, dedicated deposition chambers, the composition of the semiconductor layers being dependent upon, inter alia, the particular process gases introduced into each of the deposition chambers. The deposition chambers are connected by a relatively narrow gas gate passageway through which the substrate material passes and which is adapted to isolate the process gases introduced into the first chamber from the process gases introduced into the adjacent deposition chamber. Despite the relatively small size of the gas gate passageways, a percentage of gases introduced into one chamber still back diffuses into the adjacent chamber, thereby contaminating the layer deposited in said adjacent chamber. In an effort to reduce the flow of reacted process gases into adjacent chambers, deposition apparatus constructed by the assignee of the instant application have incorporated shields which at least partially surround the cathode region and which cooperate with introduction and evacuation conduits to inhibit the free flow of process gases from the cathode region. The process gases introduced into the cathode region are therefore directed to flow across the substrate for disassociation into plasma and subsequent deposition thereonto. However, the process gases, so introduced have been found to form flow patterns as they are deposited as semiconductor layers onto the surface of the substrate, thereby reducing the efficiency of photovoltaic devices produced therefrom. The flow pattern formation is due to the fact that certain regions of process gases flowing through the plasma region remain substantially stagnant while adjacent regions of process gases move very quickly therethrough. Since the rate of deposition onto the surface of the substrate is proportional, all other parameters being constant, to the length of time which the process gases are exposed to the electrodynamic field, any areas of localized rarification and compression cause process gases to remain in the plasma region exposed to the electrodynamic field for varying lengths of time. The result is that the process gases are deposited at different rates. Accordingly, slower moving process gases, exposed to the field for greater lengths of time, deposit film onto the substrate surface at a greater rate than the film deposited by the more rapidly moving process gases. The differences in the rates at which film is deposited onto the substrate forms the aforementioned flow patterns, said flow patterns being readily visible to the naked eye.
Further, in the deposition apparatus constructed by the assignee of this application, although each deposition chamber included a shield to direct the process gases through the plasma region to an evacuation port for withdrawal, the web of substrate was adapted to pass thereabove. Therefore, only the central section of the surface of the substrate was available for depositing semiconductor material thereonto. Accordingly, the prior art shield arrangements failed to make maximum use of the substrate surface area available for the production of semiconductor devices.
The present invention operates to substantially (1) reduce the formation of flow patterns on the layered substrate surface and (2) expose the entire surface of the substrate traveling through the plasma region of a deposition chamber for depositing thereonto semiconductor material.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques which possess (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such a technique is fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980; and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, the enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tendem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by smaller band gap materials to absorb the light passed through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltages from each cell may be added, thereby making the greatest use of the light energy passing through the semiconductor device.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, now U.S. Pat. No. 4,400,409, diled May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material.